1. Field of the Invention
The invention is in the field of wind turbines and relates to a wind turbine with a tower having an outside.
2. Description of the Related Art
Wind turbines are increasingly used for the generation of electrical energy. Principally, wind turbines consist of a tower, a nacelle located on the tower and a rotor, which is rotatably supported in the nacelle by means of a shaft. In modern wind turbines, the rotor typically has three rotor blades. The rotor blades, which have an aerodynamic profile, experience an ascending force on flowing-against by the wind and, thereby, achieve a rotational movement on the rotor. The shaft of the rotor transmits this rotational movement to a generator accommodated within the nacelle for energy generation. The transmission of the rotational movement can be arranged by means of a gearbox, in order to increase the number of revolutions provided to the generator.
During the operation of a wind turbine, sounds develop that can be found disturbing. For technical systems there are legal limitations regarding the permitted intensity of a mean noise radiation. The legal limitations in Germany, which are based on DIN-standard 2058, allow maximum values of for example 65 dB(A) in industrial regions during the day, however, merely 35 dB(A) in pure housing areas during the night. These values also have to be met by wind turbines.
Noises created by a wind turbine are, on the one hand, air-born noises that are generated by the wind striking the rotor blades and the tower and, on the other hand, noises generated by moving mechanical components (structural-born noise). The latter are typically generated in the nacelle by the generator and the gearbox, yet also ancillary equipment like cooling ventilators, pumps, as well as bearings and the azimuth drive for the rotation of the nacelle. Structural-born noises can have defined frequency peaks, which are generated for example by the frequency of interlocking teeth (meshing frequency) of the gearbox. The noises are transmitted as impact sound by existing structures and are emitted by large surfaces in the surrounding area. The tower with its comparably large outer surface forms a specifically good emitting surface. Further, the tower forms a resonator, which amplifies frequencies in the range of 1 to 500 Hz.
In order to minimize the noise, a variety of proposals have been made. For example, within U.S. Pat. No. 6,224,341 B1 (Edge Innovations & Technology), the attempt is made to reduce noise generation of moving hollow components by filling them with granular material of low density. Thereby, an internal damping is to be realized. However, this solution requires a re-design of the existing system. A different measure is taken by DE 199 30 751 A1 (Franz Mitsch). Therein, soft bearings for the gearbox and the generator should decouple these noise sources from other components and especially from the tower, in order to interrupt, thereby, the transmission path of the impact sound. Finally, it is known from U.S. Pat. No. 6,213,721 B1 (Thomson Marconi Sonar Limited) to affix metal plates on the tower wall using a damping plastic layer. The size of the plastic layer and the mass of the metal plates are to be adjusted such that the radiation of certain frequencies is reduced.
The noise reduction achieved with these proposals often does not meet the legal requirements or are not justifiable for economical reasons.
Therefore, an improved noise radiation reduction of the wind turbine is desirable, which is at the same time feasible and simple.
This object is solved by a wind turbine with a tower, having an outside, and a noise shell, which at least partially surrounds the outside of the tower and is positioned at a distance from the outside of the tower.
The noise shell which at least partially surrounds the tower serves the purpose of absorbing the noise emitted from the tower and, therefore, results in a considerable reduction of the noise emission of the wind turbine. Thereby, the noise shell forms a kind of noise cladding, which is positioned at a distance from the outside of the tower, that is an air gap remains between the wall of the tower and the noise shell. Therefore, no direct transmission path for the noise exists between the tower and the noise shell, so that the noise cannot be transmitted from the tower to the noise shell as impact sound. The wall of the tower oscillates rather freely and emits these oscillations as noise like a tower not provided with a noise shell. The noise, which is normally widely emitted is, however, intercepted by the noise shell and absorbed.
This solution has the advantage that the oscillation characteristics of the tower wall are only marginally amended. The tower can be surrounded by the noise shell at a later date, provided that a noise reduction is required.
In one embodiment of invention the noise shell comprises an outer layer and a damping layer directed towards the tower. The damping layer leads to extensive absorption of the noise. In contrast, the outer layer serves as a stabilizer of the noise shell and protects the damping layer from the effects of the weather. For example, the outer layer can be formed of glass-fiber reinforced plastic or aluminium. Preferably, the noise shell is formed with a closed, waterproof outer layer.
In a further embodiment of the invention, the noise shell is attached to the outside of the tower with a support. Thereby, the support should assure an oscillation-decoupling between the tower wall and the noise shell, that is, it should be oscillation damping. This can for example be realized by oscillation damping elements between the noise shell and the tower wall.
Often it is sufficient if the noise shell only surrounds the upper part, for example only the upper third of the tower, entirely, since the noise radiation essentially happens there.